For inhibiting corrosion of the inner and outer surfaces of metal cans, paints are conventionally applied, wherein used are thermosetting resins.
Other methods include the use of thermoplastic resins. For example, a polyolefin film is laminated on a heated tin-free steel, or a polyester film with good heat resistance is laminated on a metal sheet to be used for cans.
Many thermosetting paints are of solvent type. For forming a coating therefrom, heating at a high temperature for a long time, for example, at 150-250.degree. C. for several minutes, is necessary. In addition, a large amount of organic solvent drifts in the air during baking. Accordingly, improvements with regard to the simplification of steps and prevention of pollution are desired. It is inevitable, moreover, that a small amount of organic solvent remains in the coating when a coating is formed under the above-mentioned conditions, and the remainder of the organic solvent migrates into the food packed in a metal can applied with the coating and degrades taste and smell of the food. Also, additives or low molecular substances which may be produced by an incomplete crosslinking reaction, both being contained in the paint, may move into the food to exert the same adverse influences as in the case of residual organic solvent.
Of the above-mentioned problems, simplification of the steps and prevention of pollution can be achieved by the use of thermoplastic resins. The use of a polyester from among the thermoplastic resin films is most preferable for this end.
That is, a polyester film is superior in heat resistance, so that it does not require additives such as a heat stabilizing agent, and has less amount of low molecular substances. This film is advantageous in dealing with the problems of poor taste and smell of food caused by migration of additives or low molecular substances produced.
When a thermoplastic resin film, which is typically a polyester film, having a pinhole is laminated on a metal sheet and said laminated metal sheet is formed into a metal can, the pinhole in said film causes corrosion of the metal sheet to result, on one hand, in markedly degraded visibility or beautiful design of the trademark printed on the outer surface of the metal can, and, on the other hand, migration of oxide compound (rust) of the metal sheet to the food in the metal can, thus deteriorating taste and smell of the food, or even health of human.
As the materials for various containers of soft drinks, beer, canned food and the like, metal sheets of steel, aluminum and the like have been mainly used. Various printings and colors are applied onto the outer surface thereof to indicate the contents, brand and the like. These containers are colored and printed by such a practical method as, for example, slitting a metal sheet into a predetermined size, printing thereon by offset printing and baking; or slitting and then bending the sheet into a tubular shape, seam welding, printing by offset printing and baking. Subsequent flange processing, inside coating and baking, seam processing and so on produce metal containers.
Whether printing on a flat sheet or a tubular article after tube-forming, a direct printing on a metal material using a metal intaglio as in gravure is not available. This is because metallic materials are so hard that uniform contact of the metal intaglio with the entire printing area is extremely difficult. To overcome this difficulty, an elastic impression such as a rubber impression or flexible resin impression has been conventionally used. Printing sensitivity using such elastic intaglio is poor and clear printing cannot be achieved. In addition, complicated printing involving setting of a wide range of gradation as in half-tone printing and photograph printing is hard to perform, and only very simple printing and coloring have been actually performed.
For a gorgeous three-dimensional printing, a multiple printing using a number of paints is required. This requires an extended time of drying and baking printed inks, and once such multiple printing is incorporated in a can production process, the drying and baking of printed inks become rate-determining steps, which in turn makes the can production speed strikingly slow. The practical number of printings at an industrial scale is naturally limited, and satisfactory visibility and beautiful design cannot be produced by this type of printing.
While there are known methods for offset printing on a slit-processed metal sheet, half-tone printing is still difficult, and the situation stands the same as in the aforementioned gravure, as long as the printed matters do not have satisfactory visibility and beautiful design.
With an aim of solving the above-mentioned problems, a method for laminating a film printed with a trademark design on a metal sheet has been proposed (Japanese Patent Unexamined Publication No. 4-292942).
Generally, a thermoplastic resin film has been laminated on said metal sheet and a cured heat resistant layer is formed to prevent softening and whitening of the thermoplastic resin film due to heat for seam welding during can production and heat treatment or retort treatment after sealing the content in a container. Said cured heat resistant layer also functions to prevent incident of flaw and allows smooth passage of the sheet, in each step of can production. For example, Japanese Patent Unexamined Publication No. 5-11979 teaches that the cured heat resistant layer preferably has a static friction coefficient of not more than 0.2.
An improved slip property of the cured heat resistant layer makes the sheet pass more smoothly and prevents incident of flaw. A single improvement in slip property of the cured heat resistant layer is nevertheless insufficient to prevent flaws. The flaws on the surface of the cured heat resistant layer, which were made during can production step, food packing step and the like, partially degrade visibility of the trademark printed thereon despite the expected benefit afforded by the visibility that the printed film should have, thus failing to satisfy the high demand in the market.
When a thermoplastic resin film having a pinhole is laminated on a metal sheet and metal cans are formed from the obtained laminated metal sheet, moreover, the pinhole in said film causes corrosion of the metallic can material, which in turn vastly impairs visibility of the printed trademark and beautiful design thereof.
It is therefore an object of the present invention to provide a laminating film superior in corrosion resistance, which is devoid of a pinhole and free of the above-mentioned problems. Particularly, the object is to provide a laminating film conducive to the corrosion resistance of metal sheet due to the absence of a pinhole, which is capable of providing a beautiful metal sheet having superior visibility and high quality appearance.
Another object of the present invention is to provide a laminated metal sheet superior in corrosion resistance wherein the film is free of a pinhole.
A still another object of the present invention is to provide a metal container superior in corrosion resistance wherein the film is free of a pinhole.